ASCII methods on OsStr

Would close rust-lang#69566

I don't know enough about encodings to know if this is a valid change, however the comment on the issue suggests it could be.

This does two things:

1. Makes ASCII methods available on OsStr

2. Makes it possible to obtain a `&mut OsStr`. This is necessary to actually use `OsStr::make_ascii_*case` methods since they modify the underlying value. As far as I can tell, the only way to modify a `&mut OsStr` is via the methods I just added.

My original hope was to have these methods on `OsStrExt` for Windows, since the standard library already assumes `make_ascii_uppercase` is valid in Windows (see the change I made to windows/process.rs). If it is found these are not valid changes on non-Windows platforms, I can move the methods to the ext trait instead.

…t, r=Mark-Simulacrum

Validate git setup before accessing functionality

Closes rust-lang#70182

rename Scalar::{ptr_null -> null_ptr} and add "machine_" prefix like elsewhere

"NULL pointer" is just much more common terminology than "pointer-null".
Also I forgot two methods when renaming all the `Scalar` things to `(to|from)_machine_(u|i)size`.

Miri: use more specialized Scalar::from_ constructors where appropriate

…mulacrum

fix TryEnterCriticalSection return type

Source: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-tryentercriticalsection
Fixes rust-lang#70504